Dual zone common catch heat exchanger/chiller

ABSTRACT

Methods and systems for controlling temperatures in plasma processing chamber via pulsed application of heating power and pulsed application of cooling power. In an embodiment, fluid levels in each of a hot and cold reservoir coupled to the temperature controlled component are maintained in part by a coupling each of the reservoirs to a common secondary reservoir. Heat transfer fluid is pumped from the secondary reservoir to either the hot or cold reservoir in response to a low level sensed in the reservoir. In an embodiment, both the hot and cold reservoirs are contained in a same platform as the secondary reservoir with the hot and cold reservoirs disposed above the secondary reservoir to permit the secondary reservoir to catch gravity driven overflow from either the hot or cold reservoir.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/647,314, filed on Oct. 8, 2012, which claims the benefit of U.S.Provisional Application No. 61/552,188 filed on Oct. 27, 2011 titled“DUAL ZONE COMMON CATCH HEAT EXCHANGER/CHILLER,” the entire contents ofwhich are hereby incorporated by reference in its entirety for allpurposes. This application is related to U.S. Non-provisionalapplication Ser. No. 13/111,334 filed on May 19, 2011, titled“TEMPERATURE CONTROL IN PLASMA PROCESSING APPARATUS USING PULSED HEATTRANSFER FLUID FLOW.”

TECHNICAL FIELD

Embodiments of the present invention generally relate to plasmaprocessing equipment, and more particularly to methods of controllingtemperatures during processing of a workpiece with a plasma processingchamber.

BACKGROUND

In a plasma processing chamber, such as a plasma etch or plasmadeposition chamber, the temperature of a chamber component is often animportant parameter to control during a process. For example, atemperature of a substrate holder, commonly called a chuck or pedestal,may be controlled to heat/cool a workpiece to various controlledtemperatures during the process recipe (e.g., to control an etch rate).Similarly, a temperature of a showerhead/upper electrode or othercomponent may also be controlled during the process recipe to influencethe processing. Conventionally, a heat sink and/or heat source iscoupled to the processing chamber to maintain the temperature of achamber component at a desired temperature. To accommodate increasinglycomplex film stacks, many plasma processes expose a workpiece to anumber of sequential plasma conditions within a same processing chamber.Operations in such in-situ recipes (performed within a singlemanufacturing apparatus rather than in separately tuned systems) mayrequire temperature setpoints spanning a wide range.

SUMMARY

Methods and systems for controlling temperatures in plasma processingchamber via pulsed application of heating power and pulsed applicationof cooling power are described. In an embodiment, fluid levels in eachof a hot and cold reservoir coupled to the temperature controlledcomponent are maintained in part by a secondary reservoir shared incommon by the hot and cold reservoirs with a pump coupling each of thehot and cold reservoirs to the secondary reservoir. The pump is activelycontrolled by a level sensor in each of the hot and cold reservoirs tomaintain the heat transfer fluid level. The secondary reservoir may bedisposed below both of the hot and cold reservoirs to passively catchoverspill from either the hot or cold heat transfer loops in the eventcross-talk between the loops.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are particularly pointed out and distinctlyclaimed in the concluding portion of the specification. Embodiments ofthe invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 illustrates a schematic of a plasma etch system including a heattransfer fluid-based heat source and a heat transfer fluid-based heatsink coupled to a workpiece supporting chuck, in accordance with anembodiment of the present invention;

FIG. 2A illustrates a schematic of plumbing coupling a heat transferfluid-based heat source and a heat transfer fluid-based heat sink to aworkpiece supporting chuck, in accordance with an embodiment of thepresent invention;

FIG. 2B illustrates a cross-section schematic of a hot and a cold heattransfer fluid reservoir sharing a common secondary reservoir employedin the heat transfer fluid-based heat source/sink depicted in FIG. 2A,in accordance with an embodiment of the present invention;

FIG. 3 illustrates a method of actively maintaining level of cold andhot heat transfer fluid reservoirs, in accordance with an embodiment ofthe present invention;

FIG. 4 illustrates a block diagram of an exemplary computer systemincorporated into the plasma etch system depicted in FIG. 3, inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of embodiments of theinvention. However, it will be understood by those skilled in the artthat other embodiments may be practiced without these specific details.In other instances, well-known methods, procedures, components andcircuits have not been described in detail so as not to obscure thepresent invention. Some portions of the detailed description thatfollows are presented in terms of algorithms and symbolicrepresentations of operations on data bits or binary digital signalswithin a computer memory. These algorithmic descriptions andrepresentations may be the techniques used by those skilled in the dataprocessing arts to convey the substance of their work to others skilledin the art.

The terms “coupled” and “connected,” along with their derivatives, maybe used herein to describe structural relationships between components.It should be understood that these terms are not intended as synonymsfor each other. Rather, in particular embodiments, “connected” may beused to indicate that two or more elements are in direct physical orelectrical contact with each other. “Coupled” my be used to indicatedthat two or more elements are in either direct or indirect (with otherintervening elements between them) physical or electrical contact witheach other, and/or that the two or more elements co-operate or interactwith each other (e.g., as in a cause an effect relationship).

FIG. 1 illustrates a cross-sectional schematic view of a plasma etchsystem 300 which includes a component for which temperature iscontrolled. The plasma etch system 300 may be any type of highperformance etch chamber known in the art, such as, but not limited to,Enabler™, MxP®, MxP+™, Super-E™, DPS II AdvantEdge™ G3, or E-MAX®chambers manufactured by Applied Materials of CA, USA. Othercommercially available etch chambers may be similarly controlled. Whilethe exemplary embodiments are described in the context of the plasmaetch system 300, it should be further noted that the temperature controlsystem architecture described herein is also adaptable to other plasmaprocessing systems (e.g., plasma deposition systems, etc.) which presenta heat load on a temperature controlled component.

The plasma etch system 300 includes a grounded chamber 305. A substrate310 is loaded through an opening 315 and clamped to a temperaturecontrolled electrostatic chuck 320. The substrate 310 may be anyworkpiece conventionally employed in the plasma processing art and thepresent invention is not limited in this respect. In particularembodiments, temperature controlled chuck 320 includes a plurality ofzones, each zone independently controllable to a temperature setpointwhich may be the same or different between the zones. In the exemplaryembodiment, an inner thermal zone is proximate a center of substrate 310and an outer thermal zone is proximate to a periphery/edge of substrate310. Process gases, are supplied from gas source 345 through a mass flowcontroller 349 to the interior of the chamber 305. Chamber 305 isevacuated via an exhaust valve 351 connected to a high capacity vacuumpump stack 355.

When plasma power is applied to the chamber 305, a plasma is formed in aprocessing region over substrate 310. A first plasma bias power 325 iscoupled to the chuck 320 (e.g., cathode) to energize the plasma. Theplasma bias power 325 typically has a low frequency between about 2 MHzto 60 MHz, and in a particular embodiment, is in the 13.56 MHz band. Inthe exemplary embodiment, the plasma etch system 300 includes a secondplasma bias power 326 operating at about the 2 MHz band which isconnected to the same RF match 327 as plasma bias power 325 to providevia the RF cathode input 328 a dual frequency bias power. In one dualfrequency bias power embodiment a 13.56 MHz generator supplies between500 W and 3000 W while a 2 MHz generator supplies between 0 and 7000 Wof power for a total bias power (W_(b,tot)) of between 500 W and 10000W. In another dual frequency bias power embodiment a 60 MHz generatorsupplies between 100 W and 3000 W while a 2 MHz generator suppliesbetween 0 and 7000 W of power for a total bias power (W_(b,tot)) ofbetween 100 W and 10000 W.

A plasma source power 330 is coupled through a match (not depicted) to aplasma generating element 335 (e.g., showerhead) which may be anodicrelative to the chuck 320 to provide high frequency source power toenergize the plasma. The plasma source power 330 typically has a higherfrequency than the plasma bias power 325, such as between 100 and 180MHz, and in a particular embodiment, is in the 162 MHz band. Inparticular embodiments the top source operates between 100 W and 2000 W.Bias power more directly affects the bias voltage on substrate 310,controlling ion bombardment of the substrate 310, while source powermore directly affects the plasma density. Notably, the system componentto be temperature controlled by a temperature controller 375 is neitherlimited to the chuck 320 nor must the temperature controlled componentdirectly couple a plasma power into the process chamber. In analternative embodiment for example, a showerhead through which a processgas is input into the plasma process chamber is controlled with thetemperature controller 375. For such showerhead embodiments, theshowerhead may or may not be RF powered.

For a high bias power density (kW/workpiece area) embodiment, such asthat applicable to dielectric etching, it is problematic to supplyheating power to the chuck 320 via a resistive heater because of RFfiltering issues. For the system 300, the chuck heating power isprovided by a heat transfer fluid loop. For such embodiments, a firstheat transfer fluid loop cools the chuck 320 and a second heat transferfluid loop heats the chuck 320. In the exemplary embodiment, thetemperature controller 375 is coupled, either directly, or indirectly toa chiller 377 (heat sink) and a heat exchanger 378 (heat source). Thetemperature controller 375 may acquire the temperature setpoint of thechiller 377 or the heat exchanger (HTX) 378. A difference between thetemperature of the chiller 377 and a temperature setpoint for the chuck320 and the difference between the temperature of the heat exchanger 378and the temperature setpoint is input into a feedforward or feedbackcontrol line along with the plasma power (e.g., total bias power). Thechiller 377 is to provide a cooling power to the chuck 320 via a coolantloop thermally coupling the chuck 320 with the chiller 377. In theexemplary embodiment therefore, two coolant loops are employed. Onecoolant loop has a cold liquid (e.g., Galden or Fluorinert, etc. at atemperature setpoint of −5° C. while another loop contains liquid athigh temperature (e.g., Galden or Fluorinert, etc. at a temperaturesetpoint of 55° C.). When cooling is required a valve 385 is opened(e.g., as driven by a pulse width modulation controller 380) while avalve 386 for the heating loop is opened when heating is required. Inpreferred embodiments, only one of the heating and cooling valves 385and 386 is open at any particular time such that a total fluid flow tothe chuck 320 at any given time is delivered from either the chiller 377or the HTX 378.

In an embodiment, each of the chiller 377 and HTX 378 include areservoir (cold and hot, respectively) to contain a quantity of the heattransfer fluid that is circulated to the chuck 320 and each of the coldand hot reservoirs are fluidly coupled to a secondary reservoir 379which is to contain a quantity of the heat transfer fluid. The secondaryreservoir 379 is therefore common between both the chiller 377 and HTX378 and is to permit active leveling of the primary reservoirs in eachof the chiller 377 and HTX 378.

FIG. 2A illustrates a valve and plumbing schematic for the heat transferfluid-based heat source/sink employed in the plasma etch system of FIG.1, in accordance with an embodiment of the present invention. As furtherdepicted, a pair of heat transfer fluid supply lines 381 and 382 arecoupled to the chiller 377 and a heat transfer fluid channel embedded inthe chuck 320 (subjacent to a working surface of the chuck upon whichsubstrate 310 is disposed during processing) via the valves 385 (EV 4and EV 3, respectively). The line 381 is coupled to a heat transferfluid channel embedded subjacent to a first, outer zone, of the chuckworking surface while the line 382 is coupled to a heat transfer fluidchannel embedded subjacent to a second, inner zone, of the chuck workingsurface to facilitate dual zone cooling. Similarly, the line 381 and 382also couples the chuck 320 to the HTX 378 via the valves 386 (EV2 and EV1, respectively) to facilitate dual zone heating. Return lines 383Acomplete the coupling of each of the inner and outer zone heat transferfluid channels to the chiller 377/HTX 378 via return valves EV3 and EV1.Bypasses 383 and 384 allow for pump pressure to remain with in desiredparameters even when one or the other of the valves 386 are closed.

Each of the chiller 377 and the HTX 378 includes a primary heat transferfluid reservoir (i.e., tank or bath) which is to operate at a setpointtemperature to sink or source thermal energy. The secondary reservoir379 fluidly couples the primary heat transfer fluid reservoir in thechiller 377 to the primary heat transfer fluid reservoir in the HTX 378.As shown, the chiller 377 includes an overflow conduit 338 to pass heattransfer fluid exceeding a threshold level into the secondary reservoir379 while the HTX 378 includes a similar overflow conduit 339. In theexemplary embodiment active level control is provided by at least onepump coupled to primary fluid reservoir to transport fluid from thesecondary reservoir 379 to the primary reservoir of the chiller 377 orHTX 378. In this manner, there is no need to accurately level twoindependent reservoirs, and external connections as well as any gravityfed equalization hardware (e.g., leveling pipes, etc.) can be eliminatedto simplify system architecture. The secondary reservoir 379 isadvantageous for implementations where the operation of the valves 385and 386 (and similarly return valves EV1 and EV3) cause heat transferfluid levels in the reservoir of the chiller 377 to deviate from levelsin the reservoir of the HTX 378. This is particularly an issue where apulsed heating/cooling is utilized such that only one of valves 385, 386is open at any given time and each valve may be cycled frequently. Evenwhere the return valve EV3 or EV1 is switched in phase with the valve385 or 386, respectively, it has been found that during operation smallvariations in valve actuation rates, etc. can result in a net migrationof heat transfer fluid between the chiller 377 and HTX 378.

FIG. 2B further illustrates the secondary reservoir 379 disposed belowcold and hot heat transfer fluid reservoirs, 377A, 378A, respectively,in accordance with an embodiment of the present invention. As depicted,a low level sensor 377B is disposed in the chiller reservoir 377A, as isa low level sensor 378B for the heat exchanger reservoir 378A. Each lowlevel sensor is communicatively coupled to a pump configured to drawheat transfer fluid from the secondary reservoir 379 and dispense backto respective primary reservoir to replenish the heat transfer fluid. Asillustrated in FIG. 2C, the pump 338B is coupled to the low level sensor377B to replenish heat transfer fluid via the conduit 338A and the pump339B is coupled to the low level sensor 378B to replenish heat transferfluid via the conduit 339A. While additional pumps may be utilized todraw heat transfer liquid that from the cold and hot heat transfer fluidreservoirs, 377A, 378A in response to a high level sense and dispenseback to the secondary reservoir 379, in the exemplary embodiment boththe cold and hot heat transfer fluid reservoirs, 377A, 378A are disposedabove the secondary reservoir 379 so that overflow from either the coldor hot heat transfer fluid reservoirs, 377A, 378A will be caught in thesecondary reservoir 379. In this exemplary embodiment the overflows fromthe cold and hot heat transfer fluid reservoirs, 377A, 378A are gravityfed via overflow conduits 338, 338 into the secondary reservoir 379 as acommon catch plenum. In the exemplary embodiment, the secondaryreservoir 379 is not actively temperature controlled, such that the heattransfer fluid contained in the secondary reservoir 379 is approximatelyat ambient temperature of the environment (e.g., 25C). As furtherillustrated in FIG. 2C, the cold and hot heat transfer fluid reservoirs,377A, 378A and the secondary reservoir 379, so arranged, are allcontained within a single platform 200. Therefore, in the exemplaryembodiment illustrated in FIG. 2C, the functions and components of boththe chiller 377 and HTX 378 illustrated in FIGS. 1 and 2A are containedin the single platform 200. This integration of conventionally separatechiller and heat exchanger units simplifies leveling between the twoheat transfer fluid loops via the secondary reservoir 379.

During operation, because each of the hot and cold coolant loops istapped to control the chuck temperature, the platform 200 operates tonegate any difference in the amount of fluid which is returned from thechuck 320 to the cold and hot reservoirs 377A, 378A, respectively.Periodically, in response to the low level sensors 377B and 378B, smalldeviations between the cold and hot supply and return apportionments areequalized by fluid dispensed from the leveling pumps 338B, 339B to keepthe reservoirs filled to the appropriate levels. Of course, similarfunctionality may be achieved with a single leveling pump implemented incombination with one or more control valves actuated based on the lowlevel sensor output. Because of the relatively small fluid transferincurred by operation of the temperature control valves, the secondaryreservoir 379 places little additional load on the HX and/or Chiller,377, 378.

Returning to FIG. 1, the temperature controller 375 is to execute thetemperature control algorithms and may be either software or hardware ora combination of both. The temperature controller 375 is to outputcontrol signals affecting the rate of heat transfer between the chuck320 and a heat source and/or heat sink external to the plasma chamber305. In one feedforward embodiment, with the passage of a sample time,the current controlled temperature is acquired, the temperature setpointis acquired, and the plasma power is acquired. A temperature setpointfor the heat sink(s) may also be acquired. In the exemplary embodimentdepicted in FIG. 1, the temperature controller 375 receives a controlledtemperature input signal from the chuck temperature sensor 376 (e.g.,optical probe). The temperature controller 375 acquires a chuck setpointtemperature from a process recipe file, for example stored in the memory373, and the temperature controller 375 acquires a plasma power(measured or as set by a recipe file parameter).

FIG. 3 illustrates a method 1350 of actively maintaining level of coldand hot heat transfer fluid reservoirs, in accordance with an embodimentof the present invention. For the heater loop, the method 1350 begins atoperation 1351 with circulating the heat transfer fluid between thechamber 305 and the hot reservoir 378A of the HTX 378. If a low level issensed (e.g., by the LL sensor 378B, heat transfer fluid is drawn from acommon secondary reservoir (e.g., secondary reservoir 379) and dispensedinto the hot reservoir 378A as replenishment at operation 1371 (e.g.,for a fixed time or until a second level sensor is tripped). If the heattransfer fluid level is above the low level, the method 1350 proceeds tooperation 1361 where any excess heat transfer fluid from the HTX loop isremoved from the hot reservoir 378A, for example through overflow andgravity into the secondary reservoir 379.

The method 1350 is similar for the chiller loop, beginning at operation1352 with circulating the heat transfer fluid between the chamber 305and the cold reservoir 377A of the chiller 377. If a low level is sensed(e.g., by the LL sensor 377B, heat transfer fluid is drawn from a commonsecondary reservoir (e.g., secondary reservoir 379) and dispensed intothe cold reservoir 377A as replenishment at operation 1372 (e.g., for afixed time or until a second level sensor is tripped). If the heattransfer fluid level is above the low level, the method 1350 proceeds tooperation 1362 where any excess heat transfer fluid from the chillerloop is removed from the cold reservoir 377A, for example throughoverflow and gravity into the secondary reservoir 379.

FIG. 4 illustrates a diagrammatic representation of a machine in theexemplary form of a computer system 500 which may be utilized to performthe temperature control operations described herein, such as thecontrolling of valves in the heating and cooling loops illustrated inFIG. 1. In one embodiment, the computer system 500 may be provisioned asthe controller 370 in the plasma etch system 300 with processor 502serving as the CPU 372 and the network interface device 508 providing atleast some of the functionality represented by I/O 374 in FIG. 1. Inalternative embodiments, the machine may be connected (e.g., networked)to other machines in a Local Area Network (LAN), an intranet, anextranet, or the Internet. The machine may operate in the capacity of aserver or a client machine in a client-server network environment, or asa peer machine in a peer-to-peer (or distributed) network environment.The machine may be a personal computer (PC), a server, a network router,switch or bridge, or any machine capable of executing a set ofinstructions (sequential or otherwise) that specify actions to be takenby that machine. Further, while only a single machine is illustrated,the term “machine” shall also be taken to include any collection ofmachines (e.g., computers) that individually or jointly execute a set(or multiple sets) of instructions to perform any one or more of themethodologies discussed herein.

The exemplary computer system 500 includes a processor 502, a mainmemory 504 (e.g., read-only memory (ROM), flash memory, dynamic randomaccess memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM(RDRAM), etc.), a static memory 506 (e.g., flash memory, static randomaccess memory (SRAM), etc.), and a secondary memory 518 (e.g., a datastorage device), which communicate with each other via a bus 530.

The processor 502 represents one or more general-purpose processingdevices such as a microprocessor, central processing unit, or the like.More particularly, the processor 502 may be a complex instruction setcomputing (CISC) microprocessor, reduced instruction set computing(RISC) microprocessor, very long instruction word (VLIW) microprocessor,processor implementing other instruction sets, or processorsimplementing a combination of instruction sets. The processor 502 mayalso be one or more special-purpose processing devices such as anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA), a digital signal processor (DSP), network processor,or the like. The processor 502 is configured to execute the processinglogic 526 for performing the temperature control operations discussedelsewhere herein.

The computer system 500 may further include a network interface device508. The computer system 500 also may include a video display unit 510(e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), analphanumeric input device 512 (e.g., a keyboard), a cursor controldevice 514 (e.g., a mouse), and a signal generation device 516 (e.g., aspeaker).

The secondary memory 518 may include a machine-accessible storage medium(or more specifically a computer-readable storage medium) 531 on whichis stored one or more sets of instructions (e.g., software 522)embodying any one or more of the temperature control algorithmsdescribed herein. The software 522 may also reside, completely or atleast partially, within the main memory 504 and/or within the processor502 during execution thereof by the computer system 500, the main memory504 and the processor 502 also constituting machine-readable storagemedia. The software 522 may further be transmitted or received over anetwork 520 via the network interface device 508.

The machine-accessible storage medium 531 may further be used to store aset of instructions for execution by a processing system and that causethe system to perform any one or more of the temperature controlalgorithms described herein. Embodiments of the present invention mayfurther be provided as a computer program product, or software, that mayinclude a machine-readable medium having stored thereon instructions,which may be used to program a computer system (or other electronicdevices) to control a plasma processing chamber temperature according tothe present invention as described elsewhere herein. A machine-readablemedium includes any mechanism for storing or transmitting information ina form readable by a machine (e.g., a computer). For example, amachine-readable (e.g., computer-readable) medium includes a machine(e.g., a computer) readable storage medium (e.g., read only memory(“ROM”), random access memory (“RAM”), magnetic disk storage media,optical storage media, flash memory devices, and other non-transitorystorage media.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, while flow diagrams inthe figures show a particular order of operations performed by certainembodiments of the invention, it should be understood that such order isnot required (e.g., alternative embodiments may perform the operationsin a different order, combine certain operations, overlap certainoperations, etc.). Furthermore, many other embodiments will be apparentto those of skill in the art upon reading and understanding the abovedescription. Although the present invention has been described withreference to specific exemplary embodiments, it will be recognized thatthe invention is not limited to the embodiments described, but can bepracticed with modification and alteration within the spirit and scopeof the appended claims. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

What is claimed is:
 1. A method of controlling a temperature of a chuckin a plasma processing apparatus, the method comprising: circulating afirst heat transfer fluid at a first temperature through the chuck via afirst supply line and a first return line coupling the chuck to a firstprimary heat transfer fluid reservoir; circulating a second heattransfer fluid at a second temperature through the chuck via a secondsupply line and a second return line coupling the chuck to a secondprimary heat transfer fluid reservoir; controlling a first valvedisposed in-line with the first supply line and controlling a secondvalve disposed in-line with the second supply line; and maintaining alevel of the first primary heat transfer fluid reservoir by catchingoverflow from the first primary heat transfer fluid reservoir in thesecondary reservoir and dispensing heat transfer fluid from thesecondary reservoir to the first primary heat transfer fluid reservoirin response to sensing a low level in the first primary heat transferfluid reservoir.
 2. The method of claim 1, wherein controlling the firstvalve further comprises modulating a pulse width modulation duty cycledriving the first valve between a fully open state and a fully closedstate.
 3. The method of claim 1, wherein controlling the second valvefurther comprises modulating a pulse width modulation duty cycle drivingthe second valve between a fully open state and a fully closed state,and wherein the first valve is to be in the off state when the secondvalve is in the on state.
 4. The method of claim 1, further comprisingmaintaining a level of the second primary heat transfer fluid reservoirby catching overflow from the second primary heat transfer fluidreservoir in the secondary reservoir and dispensing heat transfer fluidfrom the secondary reservoir to the second primary heat transfer fluidreservoir in response to sensing a low level in the second primary heattransfer fluid reservoir.
 5. The method of claim 4, wherein catchingoverflow from the first primary heat transfer fluid reservoir in thesecondary reservoir further comprises collecting gravity fed runoff froma drain disposed within the first primary heat transfer fluid reservoir,and wherein catching overflow from the second primary heat transferfluid reservoir in the secondary reservoir further comprises collectinggravity fed runoff from a drain disposed within the second primary heattransfer fluid reservoir.
 6. The method of claim 5, further comprisingallowing the overflow from the first and second primary heat transferfluid reservoirs to mix within the secondary reservoir.
 7. The method ofclaim 4, wherein dispensing heat transfer fluid from the secondaryreservoir to the first primary heat transfer fluid reservoir furthercomprises pumping the heat transfer fluid through a first conduitcoupling the secondary reservoir to the first primary heat transferfluid reservoir.
 8. The method of claim 7, wherein dispensing heattransfer fluid from the secondary reservoir to the second primary heattransfer fluid reservoir further comprises pumping the heat transferfluid through a second conduit coupling the secondary reservoir to thesecond primary heat transfer fluid reservoir.
 9. The method of claim 8,wherein the pumping is performed by first and second pumps, the firstpump coupled to the first conduit and the second pump coupled to thesecond conduit.
 10. The method of claim 1, wherein dispensing heattransfer fluid from the secondary reservoir to the first primary heattransfer fluid reservoir further comprises pumping the heat transferfluid through a first conduit coupling the secondary reservoir to thefirst primary heat transfer fluid reservoir.
 11. The method of claim 10,wherein the pumping is performed by a first pump.